The present invention relates generally to electro-photographic printing and film recording apparatus, such as light emitting optical printers. More particularly, the invention relates to an efficient light emitting diode ("LED") light geometry for printing images optically by exposing a photosensitive recording medium to a light generated by light emitting diodes where the light is redirected substantially transverse to an axis of the light emitting diodes.
Optical printers make use of a variety of exposure devices such as lasers, spatial light modulators, and cathode ray tubes. All have been applied to expose photographic film as the photosensitive medium. FIG. 1 shows a prior art optical design with a light source 2 which can be any one of the aforementioned light generating devices. Light 3 is projected from the light source 2 in a conical shape where the edges of the cone have an angle .theta. relative to a central axis 9. The cone passes through a lens 4 where the central axis 9 is also the center of the lens 4 such that the cone is inverted through a spatial light modulator("SLM"), also known as a liquid crystal display, panel 5. The light 3 then passes through a projection lens 6 and is imaged onto a photosensitive medium 7. This optical system is well known in the art to have a problem in that the light has an intensity distribution over the photosensitive medium 7 according to a cos.sup.4 .theta. function. The distribution is shown in a graph 8 where the highest intensity of the light 3 is along the central axis 9 and quickly falls off towards the edges of the photosensitive medium 7. This non-uniform light distribution detrimentally affects the printed output since the image appears darker toward the edges of the photosensitive medium.
A further problem that is encountered with standard light sources 2 is with respect to the SLM panel 5. Conventional printhead designs use an SLM panel 5 that is row and column addressable and therefore each of the pixels in the SLM panel 5 are selectively turned on or turned off as is required for the particular image at any given time. A problem with this type of design is that the SLM panel 5 itself has to have electronics that can control a row and column addressing scheme for each individual pixel. As SLM panel designs get more complex, currently on the order of 1 million pixels per SLM panel, this addressing is taking up more and more valuable space on the SLM panel 5 and is becoming more and more complex. A further problem with such a design is that SLM panels are susceptible to leakage, where a pixel that is in an off state still allows some light to leak through, thereby decreasing a contrast ratio between neighboring pixels.
Another problem with conventional light sources is that, in practice, the actual radiation pattern is not the conical shape 3 that was previously described. The conical shape is a common approximation but the actual radiation pattern is an elliptical shape. Therefore, radiation projected outside the conical shape 3, which was described in the conventional optical system, is lost as unusable light, thereby decreasing the efficiency of the light source. Also, if the cone angle through the SLM is made large to increase light efficiency, then the contrast ratio of SLM will decrease.
The conventional optical system shown in FIG. 1 also illustrates another problem with currently available light sources. The light sources generally must be fixed behind a lens 4 at a distance that allows the light 3 to optimally project through the lens 4. Requiring such a distance makes the optical design more spaced out, which takes up valuable space within a printer.
Accordingly, it is an object of this invention to provide a light source for an optical printer that is compact to decrease required space within an optical printer.
It is another object of this invention to provide a light source that illuminates efficiently and uniformly so as to optimize printed output out of the optical printer.
It is still another object of the invention to simplify printhead design to allow use of less complex SLM panels in the optical printer.
It is a further object of the invention to increase contrast ratio between neighboring pixels to allow the optical printer to project sharper images onto the photosensitive medium.
These and other objects of the invention will be obvious and will appear hereinafter.